I used only a 90-135-210 for may years and it was great for me. Only replaced the 135 with 150 because I needed a larger image circle. I have grown to really like the 150, it is by far my most used lens. If you use a lot of movements a 180 might be preferred over a 150 due to image circle, but I agree that if you already have a 210 then a 180 would be too close. Id say go with a 150, its a very useful focal length.